1. Field of the Invention
The invention relates to a lightweight mirror, especially for astronomical purposes, which has a core of clear or opaque vitreous silica or high-silica glass fixedly bonded between a front plate of clear vitreous silica or high-silica glass and a back plate of clear or opaque vitreous silica or high-silica glass, the materials selected for the plates and the core having equal coefficients of thermal expansion, and it relates also to a method of making such a lightweight mirror.
2. Discussion of Prior Art
U.S. Pat. No. 2,988,959 discloses a lightweight mirror in sandwich form, which consists of a front plate and a back plate disposed at a distance therefrom, which are joined fixedly together by sections of tubing in a spaced relationship to one another. The section of tubing form the core of the mirror. Glass is used as the material for the front plate, the back plate and the tubing sections. For the construction of the mirror, the tubing sections are joined by means of a bonding agent to the one plate and then, in the same manner, to the other plate. These known mirror are simple in their construction, but they do not have sufficient rigidity against forces acting parallel to the back plate.
GB Pat. No. 1,167,895 discloses lightweight mirrors in which the front plate and the back plate are joined together by a core which consists either of tubing sections or of spacing members of cruciform cross section. Instead of the named tubing sections or spacers of cruciform cross section, spacers can also be used which consist of interlocking strips in a kind of "egg crate" construction. These lightweight mirrors consist of thermally devitrified glass (glass ceramic) which has a SiO.sub.2 content of up to 70% by weight and whose other principal components are Li.sub.2 O and Al.sub.2 O.sub.3. Instead of this thermally devitrified glass, high-silica glasses (glasses having a SiO.sub.2 content of a least 90% by weight, whose thermal expansion coefficient has been adjusted by the addition of doping agents, such as TiO.sub.2, to a value that is equal to or even smaller than the thermal expansion coefficient of vitreous silica) have been used in producing such lightweight mirrors. Such mirrors are being marketed (prospectus, "Low Expansion Materials" of Corning Glass Works, Corning, N.Y., U.S.A. 1969).
For the improvement of transverse stability, lightweight mirrors are described in GB Pat. No. 1,126,930, in which the core consists of a plate provided with through bores. Clear vitreous silica is used as the material for the mirror plate and opaque vitreous silica for the core of these lightweight mirrors. The back plate consists also of vitreous or opaque vitreous silica.
U.S. Pat. No. 3,644,022 makes known lightweight mirrors in which the core is formed of Y-shaped components which are welded together to form a honeycomb-like core framework of high rigidity. Silicon dioxide-containing material is used as the substance for these mirrors.
The lightweight mirror designs described in the last-named disclosure do have the desired adequate rigidity, especially rigidity against forces acting parallel to the back plate, but their production is exceedingly laborious and involves great risks of providing a flawed product. This becomes clear if one considers that cracks can be produced by the drilling of the holes or in the welding together of the components forming the core.